Additional information is disclosed in the following documents: International Patent Publication WO 2010/120581 A1, published Oct. 21, 2010, for “Method for Producing Solid Carbon by Reducing Carbon Oxides;” International Patent Publication WO 2013/158156, published Oct. 24, 2013, for “Methods and Structures for Reducing Carbon Oxides with Non-Ferrous Catalysts;” International Patent Publication WO 2013/158159, published Oct. 24, 2013, for “Methods and Systems for Thermal Energy Recovery from Production of Solid Carbon Materials by Reducing Carbon Oxides;” International Patent Publication WO 2013/158160, published Oct. 24, 2013, for “Methods for Producing Solid Carbon by Reducing Carbon Dioxide;” International Patent Publication WO 2013/158157, published Oct. 24, 2013, for “Methods and Reactors for Producing Solid Carbon Nanotubes, Solid Carbon Clusters, and Forests;” International Patent Publication WO 2013/158158, published Oct. 24, 2013, for “Methods for Treating an Offgas Containing Carbon Oxides;” International Patent Publication WO 2013/158155, published Oct. 24, 2013, for “Methods for Using Metal Catalysts in Carbon Oxide Catalytic Converters;” International Patent Publication WO 2013/158161, published Oct. 24, 2013, for “Methods and Systems for Capturing and Sequestering Carbon and for Reducing the Mass of Carbon Oxides in a Waste Gas Stream;” International Patent Publication WO 2014/011206, published Jan. 16, 2014, for “Methods and Systems for Forming Ammonia and Solid Carbon Products;” and International Patent Publication WO 2013/162650, published Oct. 31, 2013, “Carbon Nanotubes Having a Bimodal Size Distribution.” The entire contents of each of these documents are incorporated herein by this reference.
Methane reforming is an economical process for commercial production of hydrogen. Hydrogen may be used in the industrial production of ammonia (e.g., using the Haber-Bosch process by reaction with nitrogen gas) and as a reducing agent for numerous chemical processes, among other uses. Two processes for methane reforming are widely employed: wet methane reforming and autothermal methane reforming.
Wet methane reforming uses water and a hydrocarbon (e.g., methane), to form hydrogen and carbon monoxide (syngas):CH4+H2OCO+3H2  (1).Equation 1 is an endothermic reaction referred to in the art as the “steam-reforming” reaction. A catalyst (e.g., nickel) is typically used to promote the process. Temperatures of about 700° C. to about 1100° C. are commonly used.
A small amount of carbon dioxide is also typically formed, with additional hydrogen production:CH4+2H2OCO2+4H2  (2).At temperatures of about 130° C., as found near the outlets of smoke stacks, the carbon monoxide may further react with water:CO+H2OCO2+H2  (3).Equation 3 is commonly referred to as the “water-gas shift reaction.”
Many apparatus, methods, and improvements thereto have been developed for wet methane reforming. Autothermal methane reforming uses oxygen and either carbon dioxide or water to form hydrogen and carbon monoxide:2CH4+O2+CO23H2+3CO+H2O  (4);4CH4+O2+2H2O10H2+4CO  (5).An advantage of autothermal methane reforming over wet methane reforming is that the oxygen reacts with a portion of the methane in an exothermic reaction (combustion) and may provide heat to drive the reforming process. However, oxygen can also oxidize metal catalysts commonly used in these reactions, “poisoning” the catalyst and slowing further reactions.
In either process, most of the carbon oxides are typically oxidized to carbon dioxide, which then may either be released to the atmosphere, contributing to anthropogenic greenhouse-gas emissions, or separated from other gases and disposed of Separation of carbon oxides may involve pressure swing absorption (PSA), or other methods. Separated carbon oxides are increasingly being liquefied and may be captured, for example, by injection into oil or gas reservoirs, a costly process. Eliminating the need for separation of carbon oxides from hydrogen products of methane reforming would be of significant benefit to industry.
U.S. Pat. No. 7,794,690 (Abatzoglou et al.) teaches a dry reforming process for sequestration of carbon from an organic material. Abatzoglou discloses a process utilizing a two-dimensional (2-D) carbon sequestration catalyst with, optionally, a 3D dry reforming catalyst. For example, Abatzoglou discloses a two-stage process for dry reformation of an organic material (e.g., methane, ethanol) and CO2 over a 3D catalyst to form syngas, in a first stage, followed by carbon sequestration of syngas over a 2D carbon steel catalyst to form CNTs and carbon nanofilaments. The 2D catalyst may be an active metal (e.g., Ni, Rh, Ru, Cu—Ni, Sn—Ni) on a nonporous metallic or ceramic support, or an iron-based catalyst (e.g., steel), on a monolith support. The 3D catalyst may be of similar composition, or may be a composite catalyst (e.g., Ni/ZrO2—Al2O3) over a similar support. Abatzoglou teaches preactivation of a 2D catalyst by passing an inert gas stream over a surface of the catalyst at a temperature beyond its eutectic point, to transform the iron into its alpha phase. Abatzoglou teaches minimizing water in the two-stage process or introducing water in low concentrations (0 to 10 wt %) in a reactant gas mixture during the dry reformation first stage.
Various morphologies and allotropes of carbon are used industrially, such as for fuel, as reducing agents and electrodes in metallurgical processes, as corrosion-resistant materials in furnaces and heat exchangers, as carbon electrodes, as fillers and colorants in plastics, rubbers and inks, and as strengtheners in many polymer formulations including tires and hoses. High-purity carbon in many allotropes and morphologies is a bulk commodity chemical widely used in industry. Carbon nanotubes (CNTs) may be particularly valuable.
There is a spectrum of reactions involving carbon, oxygen, and hydrogen wherein various equilibria have been identified. Hydrocarbon pyrolysis involves equilibria between hydrogen and carbon that favors solid carbon production, typically with little or no oxygen present. The Boudouard reaction, also called the “carbon monoxide disproportionation reaction,” is the range of equilibria between carbon and oxygen that favors solid carbon production, typically with little or no hydrogen present. The Bosch reaction is within a region of equilibria where all of carbon, oxygen, and hydrogen are present under reaction conditions that also favor solid carbon production.
The relationship between the hydrocarbon pyrolysis, Boudouard, and Bosch reactions may be understood in terms of a C—H—O equilibrium diagram, as shown in FIG. 1. The C—H—O equilibrium diagram of FIG. 1 shows various known routes to solid carbon, including carbon nanotubes (“CNTs”). The hydrocarbon pyrolysis reactions occur on the equilibrium line that connects H and C and in the region near the left edge of the triangle to the upper left of the dashed lines. Two dashed lines are shown because the transition between the pyrolysis zone and the Bosch reaction zone may change with reactor temperature. The Boudouard, or carbon monoxide disproportionation reactions, occur near the equilibrium line that connects O and C (i.e., the right edge of the triangle). The equilibrium lines for various temperatures that traverse the diagram show the approximate regions in which solid carbon will form. For each temperature, solid carbon may form in the regions above the associated equilibrium line, but will not generally form in the regions below the equilibrium line. The Boudouard reaction zone appears at the right side of the triangle. In this zone, the Boudouard reaction is thermodynamically preferred over the Bosch reaction. In the region between the pyrolysis zone and the Boudouard reaction zone and above a particular reaction temperature curve, the Bosch reaction is thermodynamically preferred over the Boudouard reaction.
The use of carbon oxides as the carbon source in the production of solid carbon has largely been unexploited in methane reforming for the production of hydrogen as a way to use the carbon in the methane to produce a valuable co-product and as a way of minimizing the carbon oxides (principally carbon dioxide) typically emitted form methane reforming plants.